Cromwell Play in southeastern oklahoma

Presented in a PTTC South Midcontinent workshops in Norman, Oklahoma City and Tulsa in late 2003. Co-sponsors include the Oklahoma Geological Survey, Oklahoma City Geological Society and Tulsa Geological Society.

BOTTOM LINE

A key to understanding the Cromwell is the updated stratigraphic interpretation and nomenclature presented in the workshop. Correlation of the subsurface Cromwell sandstone to surface rocks defined as Morrow has aided construction of regional cross sections that assist in delineating the Cromwell play. This study of the Cromwell is the final workshop in a series analyzing the widespread Morrowan reservoirs in Oklahoma.

PROBLEM ADDRESSED

The Cromwell play was originally an oil play starting in the early 1900s. Recently it has become a major gas play with less potential from oil development. The large Cromwell fields located in the Arkoma Basin of Oklahoma and extending into the Arbuckle Uplift are complicated structural traps at depths of over 12,000 ft. The focus of the workshop was to evaluate the sandstone reservoirs of the Cromwell Member of the Union Valley Formation, and define the trapping mechanisms and reservoir trends to improve production potential.  

KEY WORDS:

Arbuckle Uplift, Arkoma Basin, Cromwell play, Jefferson Sandstone, Ozark Uplift, Raiford SE Field, Scipio NE Field, Union Valley Formation

SPEAKERS

Richard Andrews, Oklahoma Geological Survey, Norman, OK
 

INTRODUCTION

In the early 1900s oil was discovered in shallow Cromwell reservoirs in southeast Oklahoma. Annual production peaked at 983 MBO and 25 Bcf in 1991, but has declined to 500 MBO and 14 Bcf by 2003. Because of commingled production with other plays, recent estimates suggest that these figures may be as much as 50% off, raising gas production by 30-40 Bcf and oil by 20-30 MBO. Better understanding of the Cromwell sandstone structure and stratigraphy could lead to improved accounting, and a more accurate appraisal of the value of the Cromwell play in Oklahoma.

Regional Overview
The Cromwell sandstone is lower Pennsylvanian in age, and unconformably overlies the Springer shale, Jefferson sandstone, Goddard shale and the Caney shale progressing across the Arkoma Basin. Because the Cromwell outcrops in the western part of the Arkoma Basin, the stratigraphy is well defined. However, the divisions of the Cromwell are hard to apply in subsurface units. The Cromwell in the Arkoma Basin is equivalent to the lower part of the Morrow in the Ozark Uplift of Oklahoma and Arkansas. However, the Cromwell is predominantly sandstone, and the Morrow is mainly limestone with some sandstone. In the subsurface the Cromwell reaches up to 250 ft thick. In the northeastern part of the Arkoma Basin the Cromwell thins and becomes shale and fossiliferous limestone as it approaches the Ozark Uplift.

Cromwell Deposition and Environment
The Cromwell is considered one of the best reservoirs in Oklahoma in local areas because of it very high porosity and permeability. Well logs show porosity ranging up to 20%, and averaging around 14%. The lack of intergranular cement and the high permeability make the outcrop exposures crumbly and it is difficult to interpret bedding structures. Outcrops near Ada, OK have abundant marine trace fossils and oil shows. The depositional environment is based on cores and gamma-ray logs and regional mapping. Most of the Cromwell was deposited in high-energy, shallow marine environment, with predominantly eastward sand movement. The sand formed in broad sheets and bars extending miles from the shore in a restricted bay. In the western part of the area sand bars accumulated up to 250 ft thick, thinning to 100 ft or less in the shallower eastern part of the basin. Marine offshore bars are diagnostic of the Cromwell and are encased in marine strata above, below and laterally. Although channels in the Cromwell have not been identified in core or log data, incised channels are believed to be the mechanism for sand transport into the Arkoma Basin from the Arbuckle Uplift in the west. The workshop provided three detailed regional cross-sections of the Arkoma Basin and the Cromwell play based on 2,000 logs. Detached, shallow marine bars constitute the dominant sandstone facies in the Cromwell and also in the Morrow sandstone in the Anadarko Basin.

Case Study: Scipio NW Field
Scipio field includes separate gas pools closely spaced along upthrown fault blocks of the Cromwell and Jefferson reservoirs in a 64-section area in McIntosh County, southeastern Oklahoma. The first gas well was drilled in 1923, but significant development did not occur till the 1960s and 70s. Structural deformation in this part of the Arkoma Basin is relatively minor, but small faults provide trapping mechanisms for the gas pools. Correlations of the upper Cromwell Sandstone in Scipio field are difficult due to rapid changes in porosity and large variations in thickness of the reservoir caused by erosion. The underlying Jefferson interval is easier to correlate. The permeability and porosity can be determining by the separation between the shallow and deep resistivity curves. This is an effective technique of quickly interpreting sandstone quality for both the Cromwell and Jefferson sandstones.

Cumulative gas production form the upper Cromwell section in Scipio field is estimated at over 9 Bcf from 1971 through 2002. The estimates are based on total thickness of the units, as co-mingling with the Jefferson reservoir production makes exact figures impossible to determine. Reservoir depths range from 4,000 to 5,000 ft, and all drilling employs traditional water-based mud technology. Completion practices include acidizing with 500 to 2,000 gallons of 7.5% HCl per well. In the more productive wells fracture stimulations have been employed since the 1970s. The average cost for drilling and completion of a vertical well to 4,500 ft is $179,000. Typically the wells take 2-5 weeks to drill and are all operated by small independent companies.

Case Study: Raiford SE Field
Raiford SE Field covers a 54-section area located in south-central McIntosh County, southeastern Oklahoma. Raiford contains three gas pools that produce mainly from the upper Jefferson sandstone, Cromwell sandstone and the Hunton limestone. The pools are upthrown fault blocks. The stratigraphy is not well agreed upon by geologists and operators working in the area, and poses problems for development. The distribution of gas wells in the Jefferson conforms to the net sand isopach map of the reservoir. However, Cromwell well locations do not appear to correlate with sand thicknesses.

In 2002 there were 16 wells producing from the Cromwell and Jefferson members, and several additional wells in the field producing from younger reservoirs. The Cromwell only produces from five wells in the field. Because of multiple zone completions, co-mingling of gas is common, and determining the exact production zone is difficult. Stratigraphic cross sections for the field use the Springer shale for datum, as this unit between the Cromwell above and the Jefferson below is easy to distinguish both in core and logs. The structural deformity in the Cromwell and Jefferson is minor in this part of the Arkoma Basin. However, the minor faults are the trapping mechanisms for the different gas pools. A prominent east-west fault has a total displacement of 1,000 ft, and the minor faults branching off it have throws up to 100-150 ft.

The Cromwell sandstone at Raiford field consists of large north-south trending marine bars that join laterally forming a maximum thickness of 122 ft at depths of 4,700 ft to 5,800 ft. The Cromwell is a very clean sand with carbonate and silicate cementation, which yield resistivity differences. The technique of using deep resistivity differences to determine permeability and porosity is used at Raiford field, as at Scipio field.

Cumulative gas production from the Cromwell and Jefferson sandstones at Raiford field is estimated at 10.3 Bcf from 1961 through 2002. One of the pools has produced an estimated 96% of the recoverable gas in place, but more remains in the other two pools. Pressure has declined in the reservoir since the late 1960s. The limited areal extent of the Cromwell and the pressure decline severely limits the potential for significant future production from the field. However, proper interpretation of the stratigraphy is the key to development. Completion practices and costs at Raiford field are equivalent to those at Scipio field.

Conclusions
The workshop reviewed the problems of oil and gas production from the Cromwell Play in Oklahoma and offered updated cross-sections, stratigraphic interpretations and case studies to assist operators in defining the potential and the difficulties in developing Cromwell plays. Complex structure, past confusion with stratigraphic terms across the Arkoma Basin and correlation with units in the adjoining Ozark and Arbuckle Uplifts, and commingled production offer challenges; which small independent operators will be better able to meet using the new maps and interpretations provided by the Oklahoma Geological Survey. Special Paper #2003-2, "Cromwell Play in Southeastern Oklahoma" is available from the Oklahoma Geological Survey (phone 405-325-3031).
 

CONNECTIONS:

Richard Andrews
Oklahoma Geological Survey
100 East Boyd, Suite N131
Norman, OK 73019
Phone: 405-325-3031
Email: rdandrews@ou.edu

For information on PTTC’s South Midcontinent Region and its activities contact:
Charles Mankin, Director, Oklahoma Geological Survey
100 E. Boyd St., Room N131, Norman, OK 73019-0628
Phone 405-325-3031, Fax 405-325-7069, Email cjmankin@ou.edu

Disclaimer: No specific application of products or services is endorsed by PTTC. Reasonable steps are taken to ensure the reliability of sources for information that PTTC disseminates; individuals and institutions are solely responsible for the consequences of its use.

The not-for-profit Petroleum Technology Transfer Council is funded primarily by the US Department of Energy’s Office of Fossil Energy, with additional funding from universities, state geological surveys, several state governments, and industry donations.

Petroleum Technology Transfer Council, 16010 Barkers Point Lane, Ste 220, Houston, TX 77079
toll-free 1-888-THE-PTTC; fax 281-921-1723; Email hq@pttc.org; web www.pttc.org